A variety of resin compositions have heretofore been used as the matrices of composite materials. In the area of thermosetting resins, epoxy resins have been extensively used because, in addition to their inherent superior mechanical properties (strength and elongation, in particular), they achieve better adhesion to reinforcing materials to have them exhibit superior strength properties in the resulting composite materials than any other thermosetting resins. There is, however, a growing need to produce composite materials having even higher performance, particularly, in terms of resistance to heat, water and impact, and in order to meet this demand, the use of polyfunctional maleimides, polyfunctional cyanates or oligomers thereof, or preliminary reaction products thereof as matrix components has been reviewed. For example, a resin composition which comprises a mixture of polyfunctional maleimides and polyfunctional cyanates and an epoxy compound, as described in U.S. Pat. Nos. 4,110,364, 4,429,112 and 4,404,330, Japanese Pat. Publication No. 31279/77 and Japanese Pat. Publication (OPI) No. 184524/85 (the term "OPI" as used herein means a "published unexamined Japanese patent application") can provide a cured product excellent in various properties such as heat resistance.
However, the cured product of resin composition was still insufficient in impact resistance.
Further, a prepreg for composite material resistant to impact in which the above descibed resin composition is impregnated with carbon fiber having 1.5% or more of elongation has been proposed in Japanese Pat. Publication (OPI) No. 15439/85.
However, this does not describe compression characteristics after impact which has been established as impact resistance evaluation for composite materials, in particular for carbon fiber composite materials as primary structural material for an aircraft, and it is desired further improvement.